Strangest Programming Bugs

While working on a program (or using someone else's), what are the strangest bugs that you have ever seen?

For me:
.: I was working on a game that made use of render targets, which are essentially textures that you can render to and draw with. I was trying to work out a bug, and decided to have the texture drawn over the entire screen to see what its contents were. To my surprise, it looked like a screenshot of the title screen from an entirely different game. I had never seen any of the assets in the image before, and I had never heard of the game that was depicted. We were running on (very) old hardware (provided by our school at the time, running hardware that was barely DX8-capable), and so I thought that restarting and clearing out the VRAM would clear up the issue. I was wrong. Every time I went in, the same "screenshot" was depicted, and it was only on the computer that I was working on.

.: I was working on a text-rendering system for a different game, using C++ and the SDL this time. Everything had been going well, and the text was working just fine. I was reassigned to a different area of development, and a few days later we were getting exceptions from the font system. We used TTF_GetError and found that the SDL_ttf library wasn't initialized. I could've sworn that I had written the initialization code, especially since it was working fine after the integration, but I checked our init() function and found that the initialization for SDL_image was missing along with SDL_ttf. Somehow, we lost 2 lines of code. All of our latest changes were there, but the initialization was now gone. Not commented out, just gone.

Absolutely no idea. I ended up writing a workaround that was at least 3 times slower. I was rendering a grid in an interpreted language, where (at the time) looping was expensive on the old hardware. I was trying to render it to a target to avoid re-rendering lines that would never change, but due to the bug I just had it re-render the grid every frame.

When working on a personal project in SFML I had some issues.... For some reason, every so often, all of the graphics on screen would swap with each other. People become map tiles, map tiles became menu objects, menu objects became shadow maps, etc. I still haven't figured out why it happens, so I'm just writing off the rare occurrences as Easter eggs :p